The mechanism by which peripheral resistance is initially increased in essential hypertension is not known. Recent evidence indicates that changes in arteriolar structure and responsiveness to norepinephrine are evident before arterial blood pressure (BP) is elevated. This study will longitudinally evaluate the sequence of changes in individual areteriolar responsiveness, wall structure, and network architecture during the pre-hypertensive stage in the spontaneously hypertensive rat (SHR). Three questions will be answered by this investigation concerning differences between the SHR and normotensive control rats (NCR) between ages 3 and 7 weeks (pre-hypertensive): 1) are local hemodynamics altered prior to the rapid rise in arterial BP; 2) is there an alteration in arteriolar structure in the form of wall thickening or network changes; and 3) is arteriolar responsiveness increased? The SHR will be compared to both Wistar and Wistar-Kyoto (WKY) strains to insure an adequate control. Ten animals from each group will be evaluated at age 3, 5, and 7 weeks. The arteriolar network in the cremaster muscle will be the stie of study. Direct measurements of arteriolar pressure and flow using the micropressure and RBC velocity correlator techniques will be made to assess local hemodynamics. Measurements in vivo of arteriolar wall thickness and wall/lumen ratio will quantitate alterations in individual arteriolar wall structure. Network architectrue will be analyzed by stereometric estimation of vascular density. Responsiveness and specificity will be studied by measuring changes in lumen size and wall tension produced by iontophoretic application of four different dosages of norepinephrine and angiotensin II to the arteriole wall in the denervated muscle.